From my experience (w/ a A7N8X dlx), it seems that I can overclock
1 module few MHz higher than
2 modules (in single channel mode) few MHz higher than
2 modules (in dual channel mode)
The few overclocking MHz difference translates into higher bandwidth and 3D performance.
There is a catch that in the nforce2 controller there is a prefetch cache (this is not the L1, L2 in CPU) for memory data, and it may help to speed things up despite its lower overclock frequency. But on the other hand, if you run your system with a predetermined or fixed fsb (non-overclocker), then dual channel will give you a little bit better performance, I think due to the prefetch cache in the memory controller inside NB.
Even though dual channel has intrinsic higher bandwidth to the memory controller, but the bandwidth to the CPU is limited by the fsb bandwidth, which is the bandwidth provided by just 1 module. Unless you run a version of the MB which has integrated video that has 64x2 bus to benefit from the dual memory channel.
Also price of 1 module (512MB) is usually less than 2 module (256MB)combined. E.g. as of today, Corsair 3500C2 512MB = $162, 256MB = $89.
Even if what I said is wrong, or down the road AMD CPU fsb is twice that of memory (like current Intel P4), you can always add another module to do dual channel.
I really like to hear some DIFFERENT view points regarding to this !!!